In an analog-to-digital converter (ADC) of the folding type, the bits that form the digital output code are typically generated along two largely separate paths. A coarse conversion circuit produces a set of coarse (higher-order) bits directly in response to an analog input voltage. A fine conversion circuit produces a set of fine (lower-order) bits in response to one or more signals supplied from a circuit that folds the input voltage. See van de Grift et al, "A Monolithic 8-Bit Video A/D Converter," IEEE JSSC, Jun. 1984, pp. 374-378.
One difficulty with a folding ADC is that the bits in the coarse set sometime make transitions between binary "0" and binary "1" at values of the input voltage that are slightly different from those at which the bits in the fine set make corresponding transitions between "0" . For example, consider a simple 4-bit "binary member" code consisting of two bits supplied from the coarse converter and two bits supplied from the fine converter. Assume that the input voltage is in that portion of the input voltage range where the code is (0011). The left-most bit is the most significant bit (MSB). The right-most bit is the least significant bit (LSB).
If the input voltage is increased by an amount equivalent to one LSB, the code should change to (0100). This is not always the case. Because the coarse bits are generated along a separate path from the fine bits, phenomena such as input offset errors, delay differences, and/or noise may cause the second most significant bit to switch from "0" to "1" either before or after the two fine bits switch from "1" to "0". The code changes either to (0111) or to (0000). The actual value produced by the code is nowhere near the intended (0100). A similar problem occurs with the MSB.